In cellular communications networks, femto cell devices, also termed femto cells or simply femtos, are devices with a small coverage area and are typically deployed in homes, enterprise buildings and public places to further enhance macro-cellular services. Femto cell devices provide coverage underlying a macro cell network, for example. In this specification, the terminology femto cell device, femto cell, femto and femto base station are used interchangeably and refer to the device that provides the coverage.
Femto cell devices offer several advantages. As a femto cell coverage area is relatively small compared to that of a macro cell, data rates to end user devices may be substantially higher than those achieved via an overlying macro cell layer. This may provide improved battery life and end-user service experience. In addition, femto cells off-load end users that otherwise would use macro cells and thus improve the performance and capacity of the macro cells. By employing femto cells, the need for dual-mode handsets to support Wi-Fi and 3G technologies is eliminated because end-users can use the same 3G handset transparently in macro and femto cells.
First generation femto cell device deployment relies on static allocation of spectrum in which a portion of the total spectrum licensed to the operator is to be reserved for femto cells. This form of spectrum usage is mutually exclusive with that allocated to macro cells to ensure that carefully engineered macro cells are not impacted by femto device deployment. However, this approach is undesirable as along term solution. In several territories, especially in some European countries, the available 3G spectrum in which UMTS technologies are currently deployed is very small and is often limited to a single 5 MHz carrier required for UMTS. Thus, making such a reservation of 5 MHz carrier for UMTS femtos is either impossible or not advisable due to loss of macro-cell capacity. As air interface standards evolve to wider bands, such as for example, 20 MHz in WiMAX or for LTE (UMTS Long Term Evolution), static allocation becomes more expensive.
A solution is for femto cell devices to concurrently use the same spectrum that macro-cells use. This approach is termed “concurrent co-channel reuse” but poses significant challenges, some of which have been addressed in the context of UMTS co-channel femto cells. In addition to concurrent co-channel reuse, femto deployments may be arranged to provide exclusive access to a small subset of all subscribers. For example, for a femto deployed in a home, those handsets belonging to family members may exclusively be given permission to use the femto device. In contrast, in normal cellular deployments, typically all subscribers to a network are permitted to use every base station.
One problem with concurrent co-channel reuse is that dense deployments of femto cell devices, for example, thousands of femto cells being deployed per macro-cell, may lead to significant femto-to-macro interference and a consequent reduction in macro-cell capacity and performance. In a realistic simulation study of femto deployment in an east London suburb, it has been found that femto-to-macro interference may be controlled by appropriate power management and that the impact of dense femto deployment on macro-cell performance metrics, such call drops, may be made less significant.
Another problem arising from dense femto deployment is an associated increase in network signaling, for example, handover and location area updates, and data plane traffic, for example during handover. This arises from two design requirements. Firstly, end-user handsets should not require modification and the handsets should not distinguish between femto cell device base stations and macro-cell base stations. Secondly, femto cells should be able to be retrofitted into the legacy macro-cellular architecture by assignment of a location area code and a scrambling code.
As end user handsets are unable to distinguish between femto device base stations and macro base stations, they are also unable to determine which femto cells they are permitted to use and those from which they are excluded. Accordingly, handsets will attempt to make use of the services provided by any femto and many such attempts fail because of the exclusive access restrictions on femtos. Significant amounts of unnecessary signaling and data plane traffic are generated by handsets attempting to use excluded femtos and their subsequent denial of service.
When a user equipment (UE) attached to a macro base station on a macro network requests a handover to a femto cell, the macro network is supplied with the scrambling code of the destination femto cell. This scrambling code is usually not sufficient to uniquely identify the destination femto cell. Thus, all femto cells with that scrambling code attempt to accept the handover request. To enable them to do so, all data plane and signaling plane traffic must be sent to groups of candidate destination femto cells. This results in a large overhead in femto cell backhaul requirements and in femto cell radio resources. Every handover to a femto cell results in each femto cell with the same scrambling code receiving all the data traffic plus all the signaling traffic and each femto cell allocating radio resources in anticipation of handover. Privacy of the UE connection may also be breached, since over the air encryption keys must be shared with all candidate destination femto cells.
Currently, every femto cell base station within the geographical region covered by the macro cell ID reported by the handset attempts to accept the handover. The network forwards all the required physical layer parameters, security credentials, signaling, and data traffic to these femtos. Each femto then attempts to accept the handover. Only one will be successful, however many more have been involved with the brute-force effort.
FIG. 1 illustrates a street 1 lined with houses on both sides. The street 1 is covered by a single macro cell base station 2 with macro cell 3. Several of the houses FM1 to FM7 have a femto cell installed. The downlink radiation of femto cells is likely to leak out of the houses into the street 1. A UE traveling along the street 1 may thus detect a femto and request to handover to it. As a UE in an active call with the macro cell bases station 2 moves along the street, there is a possibility that it will attempt to handover hack kind forth (ping-pong) between the macro-cell base station 2 and at least some of the femto cells FM1 to FM7. Each handover attempt causes signaling traffic and data plane traffic to be sent to all femto cells in the area covered by the macro cell 3. The backhaul for each femto cell is loaded with potentially unnecessary traffic and the femto cell must reserve radio resources to accept the potential handover. It is possible that the UE is not permitted to access any of the femtos on the street 1, so all handover attempts in this case are a waste of resources because they will ultimately be denied.